There is growing optimism that adjusting the built environment can reverse the obesity trend through increased opportunities for physical activity and the reduced reliance on automobiles for basic transportation. The majority of research, however, on the influence of the built environment on health related outcomes have used cross-section designs. As a result, there is a limited amount of information on the causal impact of changes in the built environment on obesity levels. The proposed 24-month pilot study would provide a foundation for assessing the short-term causal influence of light rail mass transit on physical activity and obesity. The proposed study will consist of a natural experiment that examines the before and after effects of the introduction of a new light rail system in Charlotte, NC on obesity related factors. The specific aims of the proposed pilot study are: (a) to test the impact of light rail on the daily physical activity levels and BMI of riders; and (b) to assess whether the use of light rail transit leads to significant increases in physical activity and reductions in BMI of study participants who use light rail compared to similarly situated participants who do not use light rail transit. Through a two-wave longitudinal panel design of interviews with 1,000 household residents living within a mile radius of the soon-to-open light rail transit system the study will assess pre and post-levels of daily physical activity and BMI. A propensity score matching approach will be used to match light rail riders and non-riders on all relevant health, social, and economic factors. The results from the propensity score matching will be used to compare the physical activity and BMI levels over time of participants who use the light rail system with participants who do not use light rail transit. Results from the research will provide a causal test of the impact of light rail on obesity and the potential positive economic impacts of integrating public health objectives in transportation decision-making.